1. Field of the Invention
The present invention relates to a projection-type liquid crystal display apparatus for displaying an image onto a screen by enlarging and projecting an image on a liquid crystal panel by means of an optical projection system and particularly to a projection-type liquid crystal display apparatus comprising a single liquid crystal panel without any color filter.
2. Description of the Related Art
Projection-type liquid crystal apparatuses such as liquid crystal projectors and liquid crystal projection television sets have been developed for enlarging and projecting an image on a liquid crystal panel as an optical switching device by means of an optical projection system. Such liquid crystal display apparatuses include a single-panel apparatus comprising a liquid crystal panel having three color filters (CF) of blue (B), red (R) and green (G) and a triple-panel apparatus comprising monochrome liquid crystal panels each provided in optical paths of B, R and G, respectively. The single-panel apparatus has a simple configuration and reductions in size, weight and cost are easily achieved. However, it is difficult to achieve high luminance since the color filters absorb much light. Cooling of the apparatus is thereby affected as well.
In order to overcome these problems, single-panel color liquid crystal display apparatuses are disclosed in, for example, Japanese Patent Application Laid-open No. 4-60538 (1992) that corresponds to U.S. Pat. No. 5,161,042 and `Asia Display '95 (p. 887)` wherein one condenser microlens is opposed to every three pixels. Three color rays of B, R and G are entered to each microlens from mutually different directions and condensed. The light sent out from the microlens is entered to each of the three pixels corresponding to three colors of B, R and G, respectively. In the color liquid crystal display apparatus, it is possible to effectively utilize light incident on regions between pixels (black matrix regions where thin film transistors [TFf], that is, switching devices for driving pixels are formed) as well. The substantial aperture ratio (the ratio of effective pixel area to the whole pixel area) is thereby increased and high illuminance is achieved, accordingly. Since such a projection-type liquid crystal display apparatus comprises a single liquid crystal panel with a microlens array instead of a color filter, the apparatus of this type will be called projection-type liquid crystal display apparatus of the color-filterless single-panel microlens system.
FIG. 1 is a schematic view of a proposed optical system used in the projection-type liquid crystal display apparatus of the color-filterless single-panel microlens system. The apparatus comprises: a light source 501 for emitting white light; a UV-IR cut filter 502 for removing ultraviolet and infrared rays from the white light emitted from the light source 501; a glass rod integrator 503 for unifying the intensity distribution in a cross section of a bundle of rays passing through the UV-IR cut filter 502; a relay lens 504 for condensing the ray bundle sent out from the glass rod integrator 503; and a collimator lens 505 for transforming the ray bundle sent out from the relay lens 504 into a nearly parallel ray bundle. The display apparatus further comprises: dichroic mirrors 506B, 506R and 506G placed in the optical path behind the collimator lens 505 for splitting the white ray bundle sent out from collimator lens 505 into color rays of B, R and G and reflecting the color rays at angles different from one another; an incident polarizing plate 507 for transforming the color rays split by the dichroic mirrors 506B, 506R and 506G into light linearly polarized in a specific direction; a liquid crystal panel 508 for performing intensity modulation on the color rays passing through the polarizing plate 507 based on color image signals; and a projection lens 509 for condensing the light sent out from the liquid crystal panel 508 and projecting the light onto a screen 509 and composing the light.
The light source 501 is typically made up of an emitter 501a of metal-halide and a concave mirror 501b of rotation symmetry. The glass rod integrator 503 is made of glass in the shape of prism and unifies the intensity distribution in a cross section of the ray bundle incident from one end face of the integrator by reflecting the ray bundle inside a number of times and emits the ray bundle from the other end face. The liquid crystal panel 508 is a panel of the color-filterless microlens system, including pixel electrodes (not shown) regularly arranged in two dimensions in correspondence with the colors of R, G and B, condenser microlenses (not shown) each of which is opposed to every three pixel electrodes of R, G and B with a liquid crystal layer not shown in between, and an outgoing polarizing plate not shown. The condenser microlens mentioned above condenses rays of three colors B, R and G split by the dichroic mirrors 506B, 506R and 506G and entering at mutually different angles. The condenser microlens then has the rays each enter the respective pixels corresponding to the three colors of B, R and G.
In the projection-type liquid crystal display apparatus with such a configuration, spatial modulation is selectively performed on each of the rays of three colors B, R and G incident into the liquid crystal layer provided for each pixel, based on a color image signal for each color applied to each pixel electrode of the liquid crystal panel 508. The rays of color light modulated at the liquid crystal panel 508 form an image on the screen 510 by the projection lens 509 and the colors are thus synthesized. A color image is thereby projected onto the screen 510.
As described above, the projection-type liquid crystal display apparatus utilizes the glass rod integrator 503 as a means for smoothing the luminous distribution on the liquid crystal panel 508. In this case, the outgoing face of the glass rod integrator 503 is conjugated with the surface of the liquid crystal panel 508. As a result, a foreign substance such as dust deposited on the outgoing face of the integrator 503 may be enlarged and projected onto the screen 510. The quality of the image is thereby significantly reduced.
In the display apparatus, although the intensity distribution in a cross section of the outgoing ray bundle is smoothed to some degree by internal reflection of the glass rod integrator 503, some light directly reaches the outgoing face without internal reflection if the length of the integrator 503 is reduced in order to decrease the size of the apparatus as a whole. Therefore, there is limitation on smoothing the illuminance distribution on the liquid crystal panel 508. Consequently, if arc fluctuations occur in the emitter of the light source 501, the fluctuations result in flicker of the image. The image quality is thereby reduced.
In Japanese Patent Application Laid-open No. 5-346557 (1993), for example, a projection-type triple-panel liquid crystal display apparatus utilizing a multiple lens array integrator is disclosed. Instead of the rod integrator, the apparatus comprises the multiple lens array integrator made up of a first lens array wherein a plurality of lenses are arranged in two dimensions and a second lens array wherein a plurality of lenses paring up with the respective lenses of the first lens array are arranged in two dimensions.
However, the multiple lens array integrator disclosed in the publication mentioned above is particularly developed for a related-art projection-type liquid crystal display apparatus using a liquid crystal panel with color filters and a triple-panel display apparatus. The configuration of the multiple lens array integrator is therefore not applicable to the apparatus of the colorfilterless single-panel microlens system. No suggestion is made in the above-mentioned publication for such applications of the integrator, either. There are reasons as follows.
The suggested apparatus with color filters and triple-panel apparatus do not require a high degree of parallelism of light illuminating the liquid crystal panel, owing to the properties of the liquid crystal panel itself. The image quality is therefore not affected even if the incident divergence angle (or converging angle) is 14 degrees or above. The incident divergence angle is a variation range of incident angles of every ray of light incident on a specific pixel on the liquid crystal panel. However, if the incident divergence angle is too large, the outgoing divergence angle is thereby increased as well and a load applied to the projection lens becomes too heavy. The incident divergence angle is therefore typically around 14 degrees, the cost of the apparatus being considered.
As thus described, since the restriction on the incident divergence angle at the liquid crystal panel is moderate for the apparatus with color filters and triple-panel apparatus, it is possible to relatively increase the size of the second lens array that functions as a diaphragm. In the above-mentioned publication, an example of the second lens array wherein the diameter of the circumscribed circle is of the order of 70 mm is disclosed.
In contrast, the apparatus of the color-filterless single-panel microlens system performs color image display by entering three colors of B, R and G to each microlens from mutually different directions and entering the light condensed by the microlens to each of the three pixels of B, R and G, respectively. If the incident divergence angle of light illuminating the liquid crystal panel is large, one color light (B light, for example) may enter not only the pixel for B color but also a neighboring pixel (the pixel for R or G) and color mixture results. The color purity of the displayed image is thereby reduced and the quality of the image is significantly affected. It is thus required to reduce the incident divergence angle of light incident on the liquid crystal panel to a sufficiently small angle.
As thus described, the limitation on the incident divergence angle at the liquid crystal panel is specifically strict for the apparatus of the colorfilterless single-panel microlens system, compared to the apparatuses of other schemes. It is thus difficult to achieve sufficient image quality with the techniques disclosed in the foregoing publication.
Even if the strict limitation on the incident divergence angle is observed, the quantity of light reaching the liquid crystal panel is inevitably reduced, accordingly. Another problem may result that it is difficult to obtain sufficient illuminance of the image.